1. Field of the Invention
The invention pertains to the field of thermal metallurgical treating, and in particular to the annealing or spheroidizing of ferrous metals under controlled atmospheres. Ferrous metals are defined as the conventional grades of steel being denoted by grade according to the American Iron and Steel Institute (AISI) nomenclature which contain carbon and in particular to the steels conventionally designated as plain carbon, alloy steels, and alloy tool steels. As these grades of steel are raised to elevated temperature for annealing and/or spheroidizing under an ambient furnace atmosphere containing air, hydrogen, water vapor, carbon dioxide, and other chemical compounds it is well known that the surface of the steel will become reactive. Furthermore, in the presence of water vapor, hydrogen and carbon dioxide in the furnace atmosphere carbon at the surface of the steel will react and be removed from the surface. Removal of carbon from the surface promotes inhomogeniety of the cross section due to the change in chemistry and crystallography, thus changing the physical properties such as surface hardness and strength of articles which are subsequently fabricated from the ferrous metal. In the normal course the area of the metal that has been depleted of carbon must be removed by expensive finishing operations such as machining, grinding, pickling and the like.
In order to condition the plain carbon, alloy steel and alloy tool steel articles for subsequent fabricating operations it is often necessary to anneal or spheroidize the metal so that it is in its softest condition for subsequent machining, cold forging, bending, or other room temperature fabrication operations. Annealing usually encompasses heating the metal above its transition temperature so that the crystalline structure (micro structure) is that of austenite (a solid solution in which gamma iron is the solvent characterized by a face-centered cubic crystal structure), and thereafter slowly cooling the metal so that as the temperature drops below the transformation temperature a micro structure consisting of ferrite (solid solution in which alpha iron is the solvent and which is characterized by a body-centered cubic crystal structure) and carbide (a compound of carbon and iron) is formed. Very often a micro structure known as pearlite, which is a lamellar aggregate of ferrite and carbide is achieved. As the carbon content increases and sometimes the alloy content, with or without an increase in carbon content, it becomes necessary to perform a treatment called spheroidizing wherein the carbide is converted to a round or globular form to promote maximum machineability and cold working properties. Spheroidizing can take place by heating the metal to a temperature above the transformation temperature followed by a prolonged slow cooling to cause precipitation and agglomeration of the carbides, or by prolonged heating at a temperature below the transformation temperature followed by a slow cooling or oscillations of heating temperature above and below the transformation temperature for the particular ferrous metal being treated, or by austenitizing, cooling to below the transformation temperature and holding followed by slow cooling.
2. Description of the Prior Art
The prior art in regard to thermal treatment of ferrous metals under carbon controlled atmospheres is adequately summarized in the specification of U.S. Pat. No. 4,049,472, which is incorporated herein by reference.
According to the prior art, protective atmospheres for annealing and or spheroidizing can be generated by reaction of air and natural gas or other fuel gases. In order to anneal the low carbon steels (less than 0.1% carbon) a lean exothermic atmosphere formed by the combustion of the gas-air mixture is used. Water vapor can be removed from the generated atmosphere to lower the decarburizing potential of the atmosphere. Conventionally high carbon steels are annealed or spheroidized in an endothermic atmosphere generated by partially reacting a mixture of fuel gas and air in an externally-heated catalyst-filled reactor. The endothermic atmosphere may contain larger quantities of carbon monoxide and unreactive fuel which serve as carbon sources to prevent loss of carbon from the surface of the ferrous metal. It has been known that for continuous annealing and/or spheroidizing furnaces better control is achieved by mixing exothermic and endothermic gases in varying ratios to adjust the carbon potential of the furnace atmosphere to prevent or minimize decarburization of the surface of the ferrous article being annealed or spheroidized.